As demand for electrical devices are recently increased, manufacturing of semiconductor devices and wafers has increased. In general, manufacturing processes of semiconductor devices and wafers is carried out in a clean room in which a rail transport system has been used for a number of years in order to transfer objects to a target station. It has become increasingly desirable to increase transport capacity in order to raise productivity and to reduce manufacturing costs.
The conventional transport system employs a looped track and several transport vehicles traveling on the track. Since increase of transport capacity cannot be achieved using the looped track system, several branch tracks are connected to a main track to allow passing of the transport vehicles. A sliding table or a turntable for switching the main track or the branch track is provided at the branch point.
As shown in FIG. 10, a sliding-type track change mechanism 91 for a linear transport system has a sliding table 94 which is slidable between the main route 92 and the branch route 93. The sliding table 94 travels and returns on a linear guide 95 making a right angle with the main route 92 and the branch route 93. A drive screw 96 connected to a driving unit 97 is provided, and these are supported by a frame 98. The sliding table 94 is moved by the rotation of the drive screw 96.
With such a construction, the sliding table 94 is first positioned to allow the transport vehicle 99 in the main route 92 to come therein. The transport vehicle 99 passes a point A in front of the sliding table 94, travels into the sliding table 94, and stops at a point B. The sliding table 94 slides in a direction C to the branch route 93. The transport vehicle 99 comes into the branch route 93, at which point the change of route is completed.
As shown in FIG. 11, a turntable-type track change mechanism 101 for a linear transport system has a turntable 104 which pivots on a point between the main route 102 and branch route 103, and a frame 106 for supporting the turntable 104 and other parts. The turntable 104 is rotated by a drive unit 105.
With the turntable-type track change mechanism 101, a transport vehicle 107 passes a point A in front of the turntable 104, goes into the turntable 104, and stops at a point B on the turntable 104. The turntable 104 is rotated through 180 degrees to allow the transport vehicle to go to a point C. The transport vehicle 107 starts traveling on the branch route 103, at which point the change of route is completed.
As shown in FIG. 12A, the transport vehicles 99 and 107 used in the above-described mechanisms 91 and 101 have side guide rollers 111 which contact an inside wall of the track and four support wheels attached at corners of their bodies. As shown in FIG. 12B, a primary unit (stationary part) 113 of a linear motor is provided on the track, and a secondary conductor (movable part) 114 of the linear motor is provided under the bodies of the transport vehicles 99 and 107, in order to generate the motive power by magnetic induction. The transport vehicles 99 and 107 are supported in the track and move along the track.
In the conventional transport system using the above switching route mechanism, the following problems occur.
(1) Since the track change operation takes time and restricts the travel of the following transport vehicle, the time required for transportation is relatively long and the transportation capacity is decreased.
(2) When the transport vehicle accidentally goes into the sliding table or the turntable before the completion of the route change operation, the transport vehicle may be derailed.
(3) The mechanism with the movable table requires the complicated structure of the track, and the branch point becomes larger in weight and size. The sliding-type track change mechanism 91 and the turntable-type track change mechanism 101 are provided since the transport vehicles 99 and 107 travel into the branch routes 93 and 103 by means of the sliding table 94 and the turntable 104. The mechanisms are comparatively heavy and the strength of the frames 98 and 106 and the drive screw 96 must be increased. When several branch mechanisms are provided, the manufacturing cost of the transport system is increased. The driving cost of the transport system is also high because the operation of the sliding table 94 and the turntable 104 requires large amounts of power. When the track rail is attached to a ceiling, troublesome construction is needed because of load durability.
(4) Because dust may be produced in the route change operation, a suction device is additionally needed. When the transport system is provided in a clean room of a facility for manufacturing a semiconductor device, influences on the semiconductor wafers due to the dust must be avoided and movable portions should not be exposed.